[PATCH 06/14] sched: Fix migrate_disable_switch() locking

Peter Zijlstra
2025-09-10T16:00:42Z

For some reason migrate_disable_switch() was more complicated than it
needs to be, resulting in mind bending locking of dubious quality.

Recognise that migrate_disable_switch() must be called before a
context switch, but any place before that switch is equally good.
Since the current place results in troubled locking, simply move the
thing before taking rq->lock.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/core.c | 27 ++++++---------------------
1 file changed, 6 insertions(+), 21 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2344,10 +2344,10 @@ static void migrate_disable_switch(struc
if (p->cpus_ptr != &p->cpus_mask)
return;

- /*
- * Violates locking rules! See comment in __do_set_cpus_allowed().
- */
- __do_set_cpus_allowed(p, &ac);
+ scoped_guard (task_rq_lock, p) {
+ update_rq_clock(scope.rq);
+ __do_set_cpus_allowed(p, &ac);
+ }
}

void migrate_disable(void)
@@ -2702,22 +2702,7 @@ __do_set_cpus_allowed(struct task_struct
struct rq *rq = task_rq(p);
bool queued, running;

- /*
- * This here violates the locking rules for affinity, since we're only
- * supposed to change these variables while holding both rq->lock and
- * p->pi_lock.
- *
- * HOWEVER, it magically works, because ttwu() is the only code that
- * accesses these variables under p->pi_lock and only does so after
- * smp_cond_load_acquire(&p->on_cpu, !VAL), and we're in __schedule()
- * before finish_task().
- *
- * XXX do further audits, this smells like something putrid.
- */
- if (ctx->flags & SCA_MIGRATE_DISABLE)
- WARN_ON_ONCE(!p->on_cpu);
- else
- lockdep_assert_held(&p->pi_lock);
+ lockdep_assert_held(&p->pi_lock);

queued = task_on_rq_queued(p);
running = task_current_donor(rq, p);
@@ -6816,6 +6801,7 @@ static void __sched notrace __schedule(i

local_irq_disable();
rcu_note_context_switch(preempt);
+ migrate_disable_switch(rq, prev);

/*
* Make sure that signal_pending_state()->signal_pending() below
@@ -6922,7 +6908,6 @@ static void __sched notrace __schedule(i
*/
++*switch_count;

- migrate_disable_switch(rq, prev);
psi_account_irqtime(rq, prev, next);
psi_sched_switch(prev, next, !task_on_rq_queued(prev) ||
prev->se.sched_delayed);